This invention relates generally to tuning controls for communications transmitters or receivers. It particularly concerns variable rate tuning controls, i.e. those which offer a choice of either a larger or a smaller degree of change in the tuned frequency for a given rotation of the tuning control knob.
Fine tuning of communications equipment can be more easily and accurately accomplished if the tuned frequency changes slowly during a large rotation of the control knob. But such a slow rate of change becomes tedious and time consuming when large frequency changes are required; this is because many revolutions of the tuning knob are then required. The prior art has therefore developed several types of variable rate tuners, which offer a choise of tuning speeds relative to knob rotation: slower for fine tuning, and faster for large frequency changes.
One type of variable rate tuner known to the prior art provides a special manual switch for selecting the tuning speed. This approach is effective, but is also somewhat inconvenient because it requires the manipulation of two separate control devices: the speed selector switch and the tuning knob.
To provide greater convenience for the operator, the prior art has provided other variable rate turning circuits in which the rate of change of frequency with respect to a given degree of tuning knob rotation changes automatically according to the speed at which the tuning knob is turned. When the knob is turned slowly, a relatively small frequency change occurs for a given number of revolutions. But when the knob is turned rapidly, a relatively large frequency change occurs for the same number of revolutions. Special analog circuitry is provided for detecting the rotation speed of the tuning knob, and for changing the tuning rate as a function thereof. Because this circuitry is analog in nature, it tends to be sensitive to temperature and to component variations resulting from aging and other causes. It also presents problems of circuit alignment, is relatively expensive, and consumes a significant amount of power.